Also added comments, and fixing some coding style (mostly in comments too).
There are no functional changes, so no tests or documentation needed.
(This was originally part of a bugfix, but it was decided to not include this
 in that patch; instead it's done separately.)


sql/sp_head.cc:
  Renaming sp_pcontext members and methods; less cryptic and more consistent.
sql/sp_head.h:
  Renaming sp_pcontext members and methods; less cryptic and more consistent.
sql/sp_pcontext.cc:
  Renaming sp_pcontext members and methods; less cryptic and more consistent.
  Also added comments, and fixing some coding style (mostly in comments too).
sql/sp_pcontext.h:
  Renaming sp_pcontext members and methods; less cryptic and more consistent.
  Also added comments, and fixing some coding style (mostly in comments too).
sql/sp_rcontext.cc:
  Renaming sp_pcontext members and methods; less cryptic and more consistent.
sql/sp_rcontext.h:
  Renaming sp_pcontext members and methods; less cryptic and more consistent.
sql/sql_yacc.yy:
  Renaming sp_pcontext members and methods; less cryptic and more consistent.